The present invention relates to the field of oil refining, and more specifically to the field of MTBE production.
Methyl tertiary butyl ether (MTBE) is a commercially important compound useful as a gasoline blending octane component. Annual worldwide MTBE production is presently well over 14 million gallons per day.
MTBE may be commercially produced by processes well known in the art from isobutylene and methanol. Alternatively n-butene or butanes can be used in place of isobutylene, and typically a mixed C.sub.4 feedstock is used. The raffinate from such processes have relatively small quantities of oxygenated compounds including by-product dimethyl ether, unreacted methanol and MTBE product and also have other contaminants such as sulfur compounds and butadiene. The oxygenates are typically separated out from C.sub.4 raffinate in an oxygen removal unit.
Oxygen removal units of the molecular sieve adsorption type require regeneration after a period of use. Regeneration employs relatively large quantities of a regenerant such as n-butane, which can be problematic in that the amount of n-butane required may exceed that available in a typical refinery, and contamination of the n-butane during the regeneration process reduces its commercial value. In addition, contaminated n-butane, which in the past could be readily blended with gasoline, cannot be blended with gasoline in the quantities required because of new more restrictive vapor pressure regulations.
One solution presently in operation at several MTBE plants is to partially recycle the regenerant. This solution, however, is not completely satisfactory because the buildup of oxygenates and other contaminants prevents recycling of the same regenerant beyond a certain point. For example, MTBE plants which recycle n-butane regenerant still utilize at least approximately 33% of the amount of fresh n-butane which would be employed by similar plants using once-through regeneration. Further recycling of the spent regenerant has long been considered impractical because the regenerant typically has a vapor pressure which makes it difficult to separate out other components. For example, the order of pure component vapor pressures (highest to lowest) for the more important components are DME, 1,3 butadiene, n-butane, MTBE and methanol.